N-channel power MOS-FET.
Ebay/Amazon are around £12 each, RS list them as 0.79p at 5-off, but on backorder.
This is to fix a monitor PSU, but once fixed it's probably going to end up on a shelf (not even worth listing on Ebay, and even free the postage would be a killer) so I'm not desperate enough to pay the reseller fees, but keen enough to get the project off my bench that I'd buy 5 from RS if I could and give away the excess.